The skin is the largest organ in the body and serves several important functions that are essential to life. The skin retards dehydration and also acts as a barrier to the invasion of various pathogens and noxious substances. Skin is composed of the epidermis, or upper layer, and the subjacent dermis. The epidermis is the superficial layer and gives rise to the nails, hair, sebaceous glands, sweat glands, and the parenchyma of mammary glands. The epidermis is composed of several layers. As cells from the deeper layer, the stratum germinativum, move toward the surface, they begin to synthesize the intracellular protein keratin. During subsequent movement, these cells lose their distinct nuclei and form the outermost layer of the epidermis known as the stratum corneum which is comprised of several layers of cornified epidermal cells that are embedded in an intercellular matrix of semi-polar and polar lipids. This layer acts as a transport route for various drugs, such as hydrocortisone, and also acts as a barrier to the transport of other drugs and cosmetics (Elias et al., Journal of Investigative Dermatology 73:339-348, 1979). In addition, this lipid layer of the stratum corneum assists in the retention of water. The water content of the stratum corneum has a profound influence on the appearance, flexibility, texture, and dryness of the skin, and also on the absorption of drugs and other molecules into and through the skin.
With increasing age, the skin gradually loses ceramides and water and becomes drier, less flexible and supple, more wrinkled, and scaly in appearance. One of the major objectives of the cosmetic industry is to retard the drying and wrinkling of the skin that occurs with normal aging or as the result of exposure to wind, sun, cold and chemicals. As the mean age of the population increases, more people are seeking products that will retard the effects of aging on the skin and will essentially rejuvenate the skin. Increasing the moisture content of the skin is one mechanism for achieving this objective. Furthermore, enhanced skin hydration facilitates the transport of pharmaceuticals across the epidermis to reach the underlying dermis and subjacent capillaries of the lymphatic and circulatory systems.
Another objective of the cosmetic industry is to retard the drying of the hair, fingernails and toenails, which all arise from the epidermis. In addition to enhancing the youthful appearance of an individual, increased hydration of these structures prevents the painful effects of cracked nails and itchy, scaly scalp. In animals, similar problems can occur with fingernails and toenails and also with horns and hooves which are all epidermal derivatives. For example, cracked hooves can result in lameness or in injury to the coronary band, the primary growth and nutritional source for the hoof wall. Injuries to the coronary band can cause serious permanent defects in the growth of the hoof wall.
The formation of organogels containing lecithin dissolved with isopropyl palmitate or other solvents, and water, has been described by Luisi et al., Colloid and Polymer Science 268:356-374 (1990) and Scartazzini et al., The Journal of Physical Chemistry 92:829-833 (1988), which are incorporated herein by reference in their entirety. Williman et al., Journal of Pharmaceutical Sciences 81:871-874 (1992), which is incorporated herein by reference in its entirety, examined the efficacy of lecithin organogels for use in the transdermal delivery of drugs such as scopolamine and broxaterol. Williman et al., also observed that lecithin organogels had no detrimental effect on skin when compared to control samples treated with physiological saline (see page 872, column. 2, paragraph 3, Journal of Pharmaceutical Sciences 81:871-874 (1992)).
Catz et al., in U.S. Pat. No. 5,238,933, discloses skin permeation enhancer systems which increase the permeability of the skin to transdermally administered, pharmacologically active agents.
Smith, in U.S. Pat. No. 3,952,099, discloses dermatological compositions for enhancing the penetration of pharmacologically active agents, such compositions comprising a sugar ester in combination with a sulfoxide or phosphine oxide.
Loucks, in U.S. Pat. No. 4,701,471, discloses a cosmetic and pharmaceutical composition comprising bovine bone marrow acids mixed with lecithin for prevention of the fatty acid oxidation and odor putrefacation.
In U.S. Pat. No. 3,957,971, Oleniacz discloses moisturizing units for treating keratinous tissue comprising liposomes having a ternary lipid mixture of lecithin, dicetyl phosphate, and a sterol.
Sakai et al., in U.S. Pat. No. 4,760,096, discloses a skin moisturizing method and preparation containing a combination of a phosphatide such as soy lecithin, and one or more C.sub.10 -C.sub.30 carboxylic acid sterol esters.
U.S. Pat. No. 4,783,450 to Fawzi et al. discloses the use of lecithin as a skin penetration enhancer for transdermal delivery of the drug procaterol through skin.
A method and composition for increasing the moisture of epidermal structures are needed. The composition should be easy to apply topically, enhance moisture retention, and also have the capacity to deliver compounds that will assist in moisturizing and rejuvenating keratinous structures such as the skin hair, fingernails, toenails, hooves and horns.